Australia: The Land Where Time Began
New Volcanic Province – Inventory of Subglacial Volcanoes in West Antarctica
The West Antarctic Rift System is overlain by the West Antarctic Ice Sheet, and as a result of the comprehensive ice cover, there is only limited and sporadic knowledge of volcanic activity and its extent. It is important to improve understanding of subglacial volcanic activity across the province for both helping to constrain the way in which volcanism and rifting may have influenced the growth and decay of ice sheets over glacial cycles, and in light of concerns whether enhanced geothermal heat fluxes and subglacial melting may contribute to instability of the West Antarctic Ice Sheet. In this study ice sheet bed elevation data were used to locate individual conical edifices that represent subglacial volcanoes. In order to support this interpretation aeromagnetic, aerogravity, satellite imagery and databases of confirmed volcanoes were used to support this interpretation. The overall result that is presented in this paper constitutes a first inventory of subglacial volcanoes in west Antarctica. There are 138 volcanoes that were identified in this study, 91 of which have not previously been identified, and which are distributed widely throughout the deep basins of west Antarctica, though they are especially concentrated, and are oriented along, the more than 3,000 km central axis of the West Antarctic Rift System.
One of the most extensive regions of stretched continental crust on Earth is present in West Antarctica, which in dimensions and settings is comparable to the East African Rift System and the Basin and Range Province in the US (e.g. Behrendt et al., 1991; Dalziel, 2006; Kalberg et al., 2015). According to van Wyk de Vries et al. it is important to obtain improved knowledge of the geological structure of the region as it provides the template over which the West Antarctic Ice Sheet has waxed and waned over multiple glaciations (Naish et al., 2009; Pollard & DeConto, 2009; Jamieson et al., 2010), and a first order control on the spatial configuration of the ice dynamics of the WAIS (Studinger et al., 2000; Jordan et al., 2010; Bingham et al., 2012). At the present an extensive, complex network of rifts characterise the subglacial region, and this is likely to have initiated at various times since the Cainozoic (Fitzgerald, 2002; Dalziel, 2006; Siddoway, 2008; Spiegel et al., 2016), and which may still be active in some locations (Behrendt et al., 1998; LeMasurier, 2008; Lough et al., 2013; Schroeder et al., 2014). This series of rifts beneath the WAIS has collectively been termed the West Antarctic Rift System (WARS), and the Transantarctic Mountains bounds it to the south.
Rift interiors with thin, stretched crust are associated, in other major rift systems of the world, with considerable volcanism (e.g. Siebert & Simkin, 2002). In West Antarctica, however, there have been only a few studies that have identified subglacial volcanoes and/or volcanic activity (e.g. Blankenship et al., 1993; Behrendt et al., 1998, 2002; Corr & Vaughan, 2008; Lough et al., 2013), with comprehensive identification of the full spread of the volcanoes throughout the WARS being deterred by the ice cover. It is important to improve on this limited impression of this distribution of volcanoes in the WARS for several reasons:
1) The characterisation of the geographical spread of volcanic activity across the WARS can complement wider efforts to understand the main controls of rift volcanism throughout the world (Ellis & King, 1991; Ebinger et al., 2010).
2) Volcanic edifices contribute to the macroscale roughness of the ice sheet beds, by their formation of “protuberances” at the subglacial interface, and this roughness in turn forms a first-order influence on the flow of ice (cf. Bingham & Siegert, 2009).
3) Geothermal heat flow and, therefore, basal melting, also potentially impacts on the dynamics of the ice (Blankenship et al., 1993; Vogel, 2006).
4) It has been argued that the recovery of palaeoenvironmental information from the glaciations of the Quaternary, such as the thickness of the palaeoice and thermal regime, can be achieved by using the subglacial volcanic sequences.
This study presents a new assessment on a regional scale of the locations of volcanoes that are likely to be in West Antarctica, based on morphometric (or shape) analysis of the ice bed topography of West Antarctica. The shape of a volcano depends on 3 principal factors:
1) The composition of the magma that is erupted;
2) The environment into which the magma has been erupted; and
3) The erosional regime to which the volcano has been subjected since eruption (Hickson, 2000; Grosse et al., 2014; Pedersen & Grosse, 2104).
The composition of magma in large continental rifts generally has a low to medium silica content with some more alkaline eruptions (Ebinger et al., 2013). Most knowledge of volcanoes is derived from subaerial outcrops in Marie Byrd Land, West Antarctica, a region where volcanoes are composed of intermediate alkaline lavas that were erupted onto a basaltic shield, and there are smaller instances in which volcanoes are composed entirely of basalt and a few more evolved compositions (trachyte, rhyolite; LeMasurier et al., 2013). Therefore, vans Wyk de Vries et al. consider that many structures in the WARS are also basaltic. With regards to the environment of the eruption, it is typical for subaerial basaltic eruptions to produce cones that are broad shield types that protrude upwards from the surrounding landscape (Grosse et al., 2014). Monogenetic volcanoes often form steep structures that have a flat top that are comprised of phreatomagmatic deposits draped on pillow lava cores which are overlain by lava-fed deltas, termed tuyas, under subglacial conditions (Hickson, 2000; Pedersen & Grosse, 2014; Smellie & Edwards, 2016). A range of morphometries that reflect the multiple events responsible for the formation of large, polygenetic volcanic structures, though many have overall ‘conical’ structures that are similar to conical structures or shield volcanoes (Grosse et al, 2014; Smellie & Edwards, 2016).
The macrogeomorphology in the WARS is often dominated by elongated landforms that result from geologic rifting and subglacial erosion. It was proposed by van Wyk de Vries et al.in this paper that the most reasonable explanation for the presence of any ‘cones’ in this setting is that they must be of volcanic origin. They defined ‘cones’ as any features with a low length/width ratio when viewed from above; therefore, for this study, they included cones that have very low slope angles. They therefore view cones in this subglacial landscape as being diagnostic of the presence of volcanoes. They also note that identification of cones alone will not identify all volcanism in the WARS. E.g., a likely feature of rift volcanism, such as eruptions of volcanic fissures, will produce ridge forms, or ‘tindars’ (Smellie & Edwards, 2016), rather than cones. Also, in the wet basal environment of the WAIS, it is more likely that the older the cones the more likely it will have lost its conical form as a result of subglacial erosion. Cones, Therefore, that are present today are likely to be relatively young – though van Wyk de Vries et al. cannot use their method to distinguish whether or not the features are volcanically active.
Morphometry as a tool for identifying subglacial volcanoes
van Wyk de Vries et al consider in this paper implications that have arisen from their findings.
1) The approach of van Wyk de Vries et al. demonstrates that it is possible to use morphometry on the Digital Elevation Model (DEM), crucially, together with relevant auxiliary information, in order to identify potential subglacial volcanic edifices beneath West Antarctica.
2) It highlights that subglacial West Antarctica, in essence, WARS – is comprised on one of the largest volcanic provinces in the world (cf. LeMasurier et al., 1990; Smellie & Edwards, 2016), and it provides the basic metrics concerning the locations and dimensions of the main volcanic zones.
3) It highlights the wide spread of subglacial volcanism beneath the WAIS, which may have an impact on how the WAIS responds to external forcing by way of the coupling of the ice to its bed, and it may have implications for volcanic activity in the future as the ice cover thins.
According to van Wyk de Vries et al. as far as they know this study is the first to use morphometry for the identification of volcanic edifices on the continental scale beneath the Antarctic Ice Sheet. The extent of this volcanism has only been inferred by geophysical studies that were carried out previously (Behrendt et al., 2002). Morphometry has been widely used elsewhere in volcanology: E.g., to catalogue volcanic parameters, such as:
i) height, base width and crater width (e.g. McKnight & Williams, 1977; Pedersen & Grosse, 2014), or
ii) to reconstruct volcanic edifices that had been eroded (Favalli et al., 2014);
iii) to resolve volcanic characteristics in subaerial, submarine settings (e.g. Stretch et al., 2006) and
iv) Extraterrestrial (e.g. Broz et al., 2015) settings.
In all such cases, however, volcanic morphometry has been applied to DEMs that had been assembled from elevation measurements, which were distributed evenly, derived from sensors that were viewing surfaces that were unobscured. In the case of subglacial Antarctica, having confidence that the subglacial DEM, which has been constructed from elevation measurements that are non-random, is of sufficient resolution for the interpretation is key. Increasing glacial recovery of subglaciological information from morphometry has occurred over recent years. Seeding centres for glaciation of the WAIS (Ross et al., 2014) and the East Antarctic Ice Sheet (Bo et al., 2009; Rose et al., 2013) have been identified by the preponderance of sharp peaks, features that are cirque-like and valleys that are closely spaced relative to other parts of the subglacial landscape. In other places, landscapes of ‘selective linear erosion’, which are diagnostic of former dynamism in regions of ice that are now stable, have been detected from the presence of significant linear incisions (troughs) into higher surfaces that are otherwise flat (plateaux) (Young et al., 2011; Jamieson et al., 2014; Rose et al., 2014). A feature that all of these studies have in common is that they have auxiliary evidence to the morphometry that is closely considered and, therefore, have not relied only on the shape of the surface in coming to interpretations concerning formation of the landscape. It was shown by this study that such a combined approach is also valid for the locating and mapping of many volcanic edifices that were previously unknown across the ice-shrouded WARs.
Subglacial volcanism – extent and activity
At least 138 likely volcanic edifices that were distributed throughout the WARS were identified by this study. A significant advance is represented by this on a total of 47 volcanoes that had been identified across the whole of West Antarctica, most of which are visible at the surface and are situated in Marie Byrd Land and the Transantarctic Mountains (LeMasurier et al., 1990). It was noted by van Wyk de Vries et al. that the wide distribution of volcanic structures throughout the WARS, as well as the presence of clusters of volcanism that is concentrated within the Marie Byrd Land dome, shows a remarkable similarity to that of the East African Rift System, which is also more than 2,000 km long and is flanked by the Ethiopian and Kenyan domes (Siebert & Simkin, 2002; Ebinger, 2005). Morphologically, the volcanoes have characteristics of height-volume and basal diameters that match closely those of the rift volcanoes around the world. According to van Wyk de Vries et al., the total region that has undergone volcanism is likely to be considerably larger than that which has been identified by this study, keeping in mind that beneath the Ross Ice shelf the paucity of data precludes meaningful analysis of a significant terrain that is also considered to be part of the WARS.
There has been a longstanding debate concerning the activity of the WARS, with some advocating a rift that is largely inactive (LeMasurier, 2008) while others suggest volcanism on a large scale (Behrendt et al., 2002). The arguments favouring an inactive rift are based on the elevation that is anomalously low, of the WARS compared to other continental rifts (Winberry & Anandakrishnan, 2004; LeMasurier, 2008) and the relative absence of pebbles of basalt that were recovered from boreholes (LeMasurier pers. Comm., 2015). High regional heat fluxes, conversely, (Shapiro & Ritzwoller, 2004; Schroeder et al., 2014), geomagnetic anomalies (Behrendt et al., 2002) and evidence of recent subglacial volcanism (Blankenship et al., 1993; Corr & Vaughn, 2008) suggest that the rift is currently active. Evidence of a large number of subglacial volcanoes has been provided by this study, which has their quasi-conical shield type geometries still intact. The nature of the cones that are largely uneroded suggest that many may date to the Pleistocene or younger, which supports that argument that the rift is still active at present.
The results of this study do not allow a determination of whether the different volcanoes are active or not; however, the identification of multiple new volcanic edifices, as well as the improved sense of their spread and concentration across the WARS, may guide future investigation of their activity. The Marie Byrd Land massif has been studied several times previously, the findings of which suggested that massif is supported by mantle that is of particularly low density, possibly comprising a volcanic ‘hotspot’ (Hole & LeMasurier, 1994; Winberry & Anandakrishnan, 2004). Tephra layers that have been recovered from Byrd Ice Core near the WAIS divide suggest that multiple volcanoes from Marie Byrd Land were active in the late Quaternary (Wilch et al., 1999), and there is seismic activity in Marie Byrd Land that has been interpreted as volcanism that is currently active (Lough et al., 2013). Strong radar-sounded englacial reflectors in the catchment of the Pine Island Glacier have been interpreted as evidence of a volcanic eruption that occurred about 2,000-2,400 years ago (see Corr & Vaughn, 2008) while Mt Erebus on the opposite rift flank in the transarctic Mountains comprises a volcano that is known to be active that is located above another potential volcanic hotspot (Gupta et al., 2009). Across the region volcanism is also likely to contribute the elevated heat fluxes that have been inferred to underlie much of the WAIS (Shapiro & Ritzwoller, 2004; Fox Maule et al., 2005; Schroder et al., 2014). In order to recover the structure of the mantle beneath the WAIS the deployment of broadband seismics is now showing great promise (e.g. Heezel et al., 2016), and the map of van Wyk de Vries et al. of potential locations of volcanoes could help target further installations directed towards improving monitoring the subglacial volcanic activity of the continent.
Stability and future volcanism – implications
Potential influences on the stability of the WAIS is provided by the wide spread of volcanic edifices and the potential of extensive volcanism throughout the WARS. Basins that descend from sea level with distance inland, that underlie many parts of the WAIS, lend the ice sheet a geometry that is prone to runaway retreat (Bamber et al., 209; Alley et al., 2015). The likelihood that extensive retreat occurred in the WAIS during the glacial minima of the Quaternary (Naish et al., 2009) and concurrently contributed to several metres of global sea level rise (O’Leary et al., 2013). The WAIS may be currently undergoing another such wholesale retreat, as ice in the sector facing the Pacific Ocean has been retreating consistently from the time of the earliest aerial and satellite observations (Rignot, 2002; McMillan et al., 2014; Mouginot et al., 2014). van Wyk de Vries et al. do not consider it likely that volcanism has had a significant role in the triggering of the current retreat, for which there is compelling evidence that the forcing has initiated from the margins (Turner et al., 2017), but they do not propose that subglacial volcanism has the potential to influence future rates of retreat by;
1) Producing enhanced basal melting that could impact of the basal motion of the ice and
2) Providing edifices that may act to pin retreat.
Some authors have suggested, with regard to the first of these possibilities, that the active subglacial volcanism could play a role in instability of the WAIS (Blankenship et al., 1993; Vogel et al., 2006; Corr & Vaughn, 2008). Subglacial volcanism in Iceland has provided an analogy, where subglacial eruptions have been known to melt basal ice, which floods the basal interface and induces periods of enhanced ice flow (e.g. Magnússon et al., 2017; Einarsson et al., 2016); however, in the ice caps of Iceland the ice is considerably thinner than it is in the WAIS and, therefore, more prone to subglacial-induced uplift. There is, nevertheless, evidence suggesting changes to subglacial distribution of water can occur beneath the WAIS, and that they can sometimes have profound impacts on the dynamics of the ice; examples are ice-dynamic variability over subglacial lakes (e.g. Siegfried et al., 2016) or the suggestion that subglacial pulses of water may have been responsible for historical occurrences of piracy of ice streams (e.g. Anandakrishnan & Alley, 1997; Vaughn et al., 2008). There has been much recent attention on the drainage of subglacial lakes comprising plausible triggers for such dynamic changes, though subglacial eruption may represent another source of pulsed water where the occurrence is seen only rarely, if ever, has been factored into models of ice sheets. There is a potential for inactive or dormant volcanism to influence ice flow by increasing heat flux to the subglacial interface; a basal melt cavity may be produced by this and the flow of ice enhanced (Bourgeois et al., 2000; Schroder et al., 2014).
Volcanic edifices, on the other hand, stand as significant protuberances, whether active of not, which may act geometrically as stabilising influences on the retreat of the ice. It is shown by numerical models that are used to project potential rates of WAIS retreat, once initiated, as long as the ice bed is smooth and downslopes inland, retreat will continue unabated, but retreat will be delayed or stopped by any increase in roughness or obstacle in the bed (Ritz et al., 2015; Nias et al., 2016). van Wyk de Vries et al. identified in this study a number of volcanic edifices situated in the deep basins within the WAIS; these edifices which are likely to result from volcanism, could represent some of the most influential pinning points for retreating ice, past and future.
In the future, the thinning and potential removal of ice cover from the WARS volcanic province could have profound impacts for volcanic activity across the region in the future. It has been shown by research in Iceland that when the ice cover thins the production of magma increased at depth in response to decompression of the underlying mantle (Jull & McKenzie, 1996; Schmidt et al., 2013). Also, evidence has been found that, worldwide, volcanism is most frequent in deglaciating regions as the overburden pressure of the ice is first reduced and then removed (Huybers & Langmuir, 2009; Praetorius et al., 2016). Therefore, significant potential to increase partial melting and eruption rates throughout the rifted terrain is offered by unloading of the WAIS from the WARS. The concentration of volcanic edifices along the WARS could be construed as evidence that such enhanced volcanic activity was a feature of Quaternary minima. The possibility is raised by this that a thinning of the ice cover and glacial unloading over the WARS in the future may increase and this, in turn, may lead to an enhanced production of water and contribute to further potential ice-dynamical instability.
If morphometric analysis is applied to a digital elevation model of the West Antarctic Rift System, and assessment of the results with respect to auxiliary information from expressions of ice surface to aerogeophysical data, van Wyk de Vries et al. have identified 138 subglacial volcanic edifices that are spread throughout the rift. In the broad rift zone the volcanoes are distributed widely, with particular concentrations in Marie Byrd Land and along the axis of the central WARS. It is demonstrated by the results that one of the world’s largest volcanic provinces, which is on a similar scale to the East African Rift System, is shrouded by the West Antarctic Ice Sheet. Beneath West Antarctica the overall density of volcanoes was found to be 1 edifice per 18,500 ± 500 km2, with a central belt along the central sinuous ridge that contains 1 edifice per 7,800 ± 400 km2. The presence of such a volcanic belt that traverses the deep marine basins beneath the centre of the WAIS could prove to be a major influence of behaviour in the past and the stability of the ice sheet in the future.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|